Institute of Electrical and Electronics Engineers having its headquarter in the US has established IEEE802.11b standard which typically represents wireless local-area-network (hereinafter referred to as LAN) devices having been rapidly and widely used. On top of that, audio-video (hereinafter referred to as AV) devices and personal computers are connected in wireless manner, so that a society where seamless networks are established can be predicted. This background urgently needs a technique embodying a compact and high-speed wireless data device at an inexpensive cost. A distance measuring technique employing wireless techniques including ultrasonic sensors, or millimeter-wave sensors is one of the applications of the communications using this standard, and this technique is used in a variety of fields such as preventing a car collision, monitoring an invader by detecting a human-body. The application fields will be expanded from now on.
A wireless method called ultra wide band (hereinafter referred to as UWB), which employs pulse-like modulating signals, has drawn attention as one of techniques that can embody a compact device, high-speed data communication, and highly accurate distance measuring. The UWB employs a short pulse not greater than 1 (one) nano-second, and a wide frequency band over several hundreds of MHz for communication or distance measuring. In the communication, a repetition frequency of the short pulse is raised, so that one symbol is assigned to each one of short pulses or a plurality of short pulses, thereby embodying a high speed communication over several hundreds of Mbps. In the distance measuring, an arrival time is measured by using short pulses, so that a distance can be accurately measured.
FIG. 9 shows a block diagram illustrating a structure of a conventional receiving apparatus. Since the UWB employs an extraordinary short pulse not greater than 1 ns, a capture of a signal (acquisition) and a synchronization of a signal (tracking) should be done quickly and accurately. In this conventional instance, a signal is captured by a technique called synchronous addition which also suppresses errors in synchronization. A signal received via a wireless propagating route is deteriorated its signal-to-noise (S/N) ratio due to adverse effect of attenuation, noise, and multi-path.
To overcome this problem, a reception signal is delayed at known pulse intervals by using plural delay circuits 701A, 701B, and the delay is added in the form of voltage by using voltage adding circuit 702 for boosting the signal voltage. A sync pulse is extracted from this signal by using sync pulse detecting circuit 703. The reception signal is demodulated with this extracted sync pulse by demodulating circuit 704, thereby obtaining a demodulated signal (demodulated data). The foregoing structure is disclosed in, e.g. Unexamined Japanese Patent Publication No. H05-284128.
Although a description by using drawings is omitted here, use of a spread spectrum technique allows to a receiving apparatus to improve the S/N ratio of a signal sent from a transmitting apparatus due to spectrum gain for reception (demodulation). This conventional structure is disclosed in, e.g. Unexamined Japanese Patent Publication No. 2003-143109.
However, the conventional transmitting apparatus, receiving apparatus and communication system discussed above sometime encounter the following problems: Synchronous addition adds not only a reception signal but also noise power, so that improvement effect of S/N ratio is lowered. Use of the spread spectrum technique takes a time for synchronizing spectrum signals, so that an actual throughput can be lowered. On top of that, a number of circuits for synchronizing the spectrum signals are needed, which makes the circuit complicated. As a result, the device becomes bulky, and the power consumption becomes greater.